Electronic ballasts typically include an inverter that provides high frequency current for efficiently powering gas discharge lamps. Inverters are generally classified according to switching topology (e.g., half-bridge or push-pull) and the method used to control commutation of the inverter switches (e.g., driven or self-oscillating). In many types of electronic ballasts, the inverter provides a square wave output voltage. The square wave output voltage is processed by a resonant output circuit that provides high voltage for igniting the lamps and a magnitude-limited current for powering the lamps.
When the lamps fail, are removed, or otherwise cease to operate in a normal fashion, it is highly desirable that the inverter be shut down or at least shifted to a different mode of operation. This is necessary in order to minimize power dissipation, reduce heating in the ballast, and protect the inverter transistors from damage due to excessive voltage, current, and heat. Circuits that shut down or alter the operation of the inverter in response to lamp removal or failure are customarily referred to as inverter protection circuits.
Several existing types of protection circuits utilize a current path through the lamp filaments to detect lamp removal or failure. This approach alone is inadequate for those situations in which a lamp fails to operate in a normal manner, but its filaments remain intact, such as what occurs with "degassed" and "diode mode" lamps. Furthermore, if multiple lamps are present, and if a single current path through the filaments of all the lamps is used, the inverter may be shut down even if only one filament of a given lamp fails but the remaining lamps are operational and with their filaments intact. This is unnecessary and undesirable, since it is preferred to have the inverter continue to operate so that the remaining operational lamps may continue to provide illumination, thus obviating any urgent need for replacement of the single failed lamp.
Many existing inverter protection circuits respond to a lamp fault condition by shutting down the inverter and then keeping the inverter off for as long as power is applied to the ballast. With such protection circuits, after replacement of a failed lamp with an operational lamp, it is required that power to the ballast be turned off and then on again (i.e., "cycled") in order to effect ignition and powering of the lamps in the fixture which was relamped. This limitation poses a considerable inconvenience in many environments, such as offices and factories, in which a large number of ballasts are often connected in the same branch circuit. In such environments, it is often necessary to momentarily interrupt the lighting in a large area in order to restore desired operation to even a single lighting fixture after one or more of its lamps are replaced.
It is therefore apparent that a need exists for an inverter protection circuit that provides protection of the inverter switches and other ballast components under various lamp failure modes, such as lamp removal or a degassed lamp, and that also allows the inverter to continue to operate when at least one operational lamp is present and when the failed lamps present no danger to the inverter. In addition, a need exists for a relamping circuit that, following lamp replacement, provides ignition and powering of the lamps in an automatic manner and without any need for cycling the power to the ballast.
A number of protection circuits attempt to detect diode-mode behavior of a lamp by monitoring the voltage across one or more components of the resonant output circuit. For example, a common approach is to unidirectionally monitor the voltage across the resonant capacitor, wherein an abnormally high positive voltage is interpreted as an indication of diode-mode behavior. A drawback of this approach is that it fails to detect those cases in which a diode-mode lamp causes a negative overvoltage condition, but not a positive overvoltage condition, across the resonant capacitor. Thus, a need exists for a detection circuit that more completely provides detection of diode-mode behavior by monitoring the output circuit for both positive and negative overvoltage conditions.
Many existing protection circuits require a large number of discrete components. This makes the ballast physically large, materially expensive, and difficult to manufacture. It is highly desirable to have a protection circuit that employs only a modest amount of discrete circuitry and that incorporates the greater portion of the protection circuitry in an inverter control circuit that is suited for implementation as an integrated circuit. Such a protection circuit would significantly enhance the reliability and manufacturability of the resulting ballast and would thus represent a considerable advance over the prior art.